Eyepieces for medical and non-medical applications generally allow for positioning or displacement of the area perceived as in focus by the relaxed human eye and for a correction of axial ametropia (in particular hyperopia or farsightedness and myopia or nearsightedness) of an eye of a user of the eyepiece. Both parts together are commonly referred to as focusing and this is generally brought about by displacing the whole eyepiece or parts of the eyepiece parallel to the optical axis.
However, in addition to axial ametropia, the eyes of very many people exhibit an astigmatism. Due to a non-rotationally symmetric curvature of the cornea, a non-rotationally symmetric shape of the lens, an uneven contraction of the ciliary muscle and/or for different reasons, light rays emanating from an observed punctiform object are not imaged on the retina on a point, but rather in a line. Particularly the cornea and the lens of the eye have different refractive powers in different directions or in different planes perpendicular to the optical axis. The difference between the maximum refractive power and the minimum refractive power is specified in diopter. In order to characterize an astigmatism, the orientation of the plane of maximum refractive power or the orientation of the plane of minimum refractive power is furthermore specified; the two axes are orthogonal to one another in that significantly more common regular astigmatism.
Persons with not only a small astigmatism generally prefer not to remove their spectacles when looking through an eyepiece. However, this presumes a suitable design of the eyepiece. Moreover, the spectacles can be dirtied or the often high-quality, but mechanically sensitive coating thereof may be damaged in the case of the almost unavoidable contact with the eyepiece. As an alternative, some eyepieces have cylinder lenses which can be inserted or placed thereon. However, the eyepiece can then still only be used by a person or persons who perchance have the same astigmatism, and said persons can also only use said eyepiece without spectacles. These additional lenses are out of the question for practical reasons if an eyepiece is intended to be used by different persons or, alternately, with and without spectacles.